Lithium secondary batteries have been in practical and widespread use as secondary batteries for portable electronic devices. Furthermore, in recent years, lithium secondary batteries have drawn attention not only as small-sized secondary batteries for portable electronic devices but also as high-power and high-capacity devices for use in vehicles, power storage, and the like. Therefore, there has been a growing demand for the lithium secondary batteries with higher safety standards, lower costs, longer lives, and the like.
A lithium secondary battery is composed mainly of a cathode, an anode, an electrolyte, a separator, and an armoring material. Further, the cathode is constituted by a cathodic active material, a conductive material, a current collector, and a binder (binding agent).
In general, the cathodic active material is realized by a layered transition metal oxide such as LiCoO2. However, in a state of full charge, such layered transition metal oxides are prone to cause oxygen desorption at a comparatively low temperature of approximately 150° C., and such oxygen desorption may cause a thermal runaway reaction in the battery. Therefore, when a battery having such a cathodic active material is used for a portable electronic device, there is a risk of an accident such as heating, igniting, and the like of the battery.
For this reason, in terms of safety, expectations have been placed on lithium manganate (LiMn2O4) having a spinel-type structure, lithium iron phosphate (LiFePO4) having an olivine-type structure, and the like that are stable in structure and do not emit oxygen in abnormal times.
Further, in terms of cost, cobalt (Co) is low in degree of existence in the earth's crust and high in price. For this reason, expectations have been placed on lithium nickel oxide (LiNiO2) or a solid solution thereof (Li(Co1−xNix)O2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4), and the like.
Therefore, for example, such lithium iron phosphate having an olivine-type structure has drawn attention as a cathodic active material for a battery considering the safety, cost, and battery life. However, when the lithium iron phosphate having an olivine-type structure is used as a cathodic active material for a battery, there is a problem in that rate performance is low. That is, there is a problem in that, along with an increase in load discharge (current discharge), the discharge capacity and the discharge voltage greatly deteriorate due to an increase in the internal resistance of a battery.
In order to solve this problem, PTL 1 discloses a method of replacing P site of a cathodic active material with element A to increase the conductivity of the cathodic active material and improve the discharge capacity, in which the cathodic active material is represented by Formula LiMP1−xAXO4 (wherein M is a transition metal, A is an element having an oxidation number of +4 or less, and 0<x<1).
In addition, PTL 2 discloses a method of using a material represented by Formula Li1−xAxFe1−Y−ZMyMezP1−mO4−nZn (wherein A is at least one of Na and K; M is at least one of metal elements other than Fe, Li, and Al; Me is at least one of Li and Al; X is at least one of Si, N, As, and S; Z is at least one of F, Cl, Br, I, S, and N; 0≦x≦0.1; 0≦y≦0.5; 0≦z≦0.3; 0≦y+z≦0.5; 0≦m≦0.3; 0≦n≦0.5; and x+z+m+n>0) as a cathodic active material to improve large-current charge-discharge behavior.
In addition, PTL 3 discloses a method of using a material represented by Formula Aa+xMbP1−xSixO4 (wherein A is selected from the group consisting of Li, Na, K, and mixtures thereof, where 0<a<1 and 0≦x≦1; and M comprises one or more metals, comprising at least one metal which is capable of oxidation to a higher valence state, where 0<b≦2) as a cathodic active material to increase capacity, cycling performance, and reversibility and to reduce cost.